This invention relates to a process for producing a heat-resistant, ferritic stainless steel sheet, and particularly to a process characterized by applying low temperature hot rolling to a Nb-containing ferritic stainless steel with a high Si content.
Heat-resistant ferritic stainless steel has been used for producing a variety of high-temperature articles such as heating apparatuses (e.g. oil stoves and kerosine stoves), kitchen utensils, exhaust gas converters, heat-exchanging apparatuses, and boilers. For manufacturing high-temperature articles used for a purpose in which a high level of mechanical strength is not necessary, or in which the smaller the degree of thermal expansion is, the more preferable, a ferritic stainless steel is used in preference to an expensive, heat-resistant austenitic stainless steel.
Some typical ferritic stainless steels are AISI Type 430 steel, AISI Type 446 steel, Fe-Cr-Al steel (JIS SUH21 steel, "Kanthal", trade name), and Cr-Si-Al steel ("Sicromal" steel, trade name, corresponding to DIN 4724). As a material for manufacturing heating apparatuses such as oil stoves, kerosine stoves, stove chimneys, ducts therefor, AISI Type 430 steel has been widely used. However, it has a maximum service temperature of 800.degree. C. When it is heated to a temperature higher than 800.degree. C., abnormal oxidation rapidly occurs. In addition, it is known that 430 type steel easily gives a red oxide scale when it is heated to a temperature of around 600.degree. C. in a combustion gas atmosphere, resulting in an oxidational metal loss, a decrease in combustion efficiency and a remarkable deterioration in appearance.
Fe-Cr-Al steels have sometimes been employed for the use described above. However, these steels employ expensive alloying elements. In addition, hot rolling of these steels does not produce a significant improvement in toughness. Namely, hot-rolled sheets of these steels exhibit a high ductile-brittle transition temperature. Therefore, it is necessary to apply warm rolling in place of cold rolling. This warm rolling is carried out at a temperature of around 150.degree. C., for example, in order to produce a thin steel sheet and is more costly than cold rolling. Due to the cost of alloying elements and the difficulties involved in manufacturing the steel sheet, the resulting steel sheet is relatively expensive.
The same thing can be said on said Type 446 and Sicromal steels as far as the difficulties involved in manufacturing steel sheet are concerned.
Thus, there has been a great demand for a less expensive ferritic stainless steel which exhibits improved heat resistance, and particularly high-temperature oxidation and corrosion resistance.
Furthermore, since the use of high-sulfur oil has recently been increasing and that high temperature combustion, i.e. combustion in which metal components are heated to higher than 800.degree. C. has recently been required for oil and kerosine stoves, there is all the more a need for a low-cost stainless steel exhibiting improved resistance to corrosion at high temperatures.
Such high-temperature articles as already mentioned are shaped through forming operation such as press-forming. Therefore, the steel sheet used for these purposes must have excellent formability as well as high-temperature oxidation and corrosion resistance.